1. Field of the Invention
This invention relates generally to the fabrication of a giant magnetoresistive (GMR) magnetic field sensor in a magnetic read head, more specifically to a spin valve type of GMR sensor of the bottom spin valve type.
2. Description of the Related Art
Early forms of magnetic read heads decoded magnetically stored data on media such as disks and tapes by making use of the anisotropic magnetoresistive effect (AMR) in magnetic materials such as permalloy. This effect was the change in the electrical resistance, r, of certain magnetic materials in proportion to the angle between the direction of their magnetization and the direction of the current flow through them. Since changing magnetic fields of moving magnetized media, such as magnetically encoded tapes and disks, will change the direction of the magnetization in a read head, the resistance variations of the AMR effect allows the information on such encoded media to be sensed and interpreted by appropriate circuitry.
One shortcoming of the AMR effect was the fact that it produced a maximum fractional resistance change, Dr/r (where Dr is the change in resistance between the magnetic material subjected to its anisotropy field, Hk, and the material subjected to zero field), which was only on the order of a few percent. This made the sensing process difficult to achieve with accuracy.
In the late 1980""s and early 1990""s the phenomenon of giant magnetoresistance (GMR) was discovered and soon applied to read head technology. The GMR effect derives from the fact that thin (xe2x89xa120 angstroms) layers of ferromagnetic materials, when separated by even thinner (xe2x89xa110 angstroms) layers of conductive but non-magnetic materials, will acquire ferromagnetic (parallel spin direction of the layers) or antiferromagnetic states (antiparallel spin direction of the layers) by means of exchange interactions between the spins. As a result of spin dependent electron scattering as electrons crossed the layers, the magnetoresistance of such layered structures was found to be significantly higher in the antiferromagnetic state than the ferromagnetic state and the fractional change in resistance was much higher than that found in the AMR of individual magnetic layers.
Shortly thereafter a version of the GMR effect called spin valve magnetoresistance (SVMR) was discovered and implemented. In the SVMR version of GMR, two ferromagnetic layers such as CoFe or NiFe are separated by a thin layer of electrically conducting but non-magnetic material such as Cu. One of the layers has its magnetization direction fixed in space or xe2x80x9cpinned,xe2x80x9d by exchange anisotropy from an antiferromagnetic layer directly deposited upon it. The remaining ferromagnetic layer, the unpinned or free layer, can respond to small variations in external magnetic fields such as are produced by moving magnetic media, (which do not affect the magnetization direction of the pinned layer), by rotating its magnetization direction. This rotation of one magnetization relative to the other then produces changes in the magnetoresistance of the three layer structure.
The spin valve structure has now become the implementation of choice in the fabrication of magnetic read head assemblies. The trend in recent patents has been to improve the sensitivity and stability of these spin valves by novel choices of the materials used to form their various ferromagnetic and antiferromagnetic layers, by variations in the number and dimensions of such layers and by choices of the structure and composition of the leads connecting the spin valve to the external circuitry. In this connection, Kanai (U.S. Pat. No. 5,896,252) teaches a method for constructing a spin valve magnetoresistive (SVMR) head element in which the free (unpinned) magnetic layer is manufactured in a two-layer structure composed of a CoFe layer and an NiFe layer. Barnard et al. (U.S. Pat. No. 5,919,580) teach a method of forming a spin valve device (SVMR) whose antiferromagnetic pinning layer is chromium rich and has a tunable Neel temperature and anisotropy constant. Fontana, Jr. et al. (U.S. Pat. No. 5,701,223) teaches a method of constructing a SVMR sensor that uses a laminated antiparallel (AP) pinned layer in combination with an improved antiferromagnetic (AF) layer. Pinarbasi (U.S. Pat. No. 5,883,764) teaches a method of providing an SVMR structure with a very thin and highly conductive lead structure.
Improvements in the design and fabrication of SVMR read heads must now be directed towards their use in decoding hard disks whose magnetic information content is approaching a density of 20 gigabytes per square inch (20 Gb/in2). As presently fabricated, SVMR sensors are adequate for densities on the order of a few gigabytes per square inch, but they lack the physical properties necessary to accurately decode the increased density. Kamiguchi et al., in xe2x80x9cCoFe Specular Spin Valves With A Nano Oxide Layer,xe2x80x9d a paper presented at the 1999 Intermag Conference, outline a method of forming a spin valve structure with an enhanced GMR ratio. Their structure makes use of a CoFeO/TaO specularly reflecting layer, a CoFe free layer and an IrMn antiferromagnetic layer. It is the aim of the present invention to also address the problem of fabricating an SVMR read head that is capable of decoding ultra-high densities of magnetically encoded information.
A first object of this invention is to provide a method for forming a magnetoresistive (MR) sensor element whose operation is based upon the giant magnetoresitive properties of certain magnetic structures, along with the magnetoresistive (MR) sensor element whose operation is so based.
A second object of this invention is to provide a method for forming a magnetoresistive (MR) sensor element which is capable of and suitable for decoding ultra-high density (20 Gb/in2) magnetic recordings, along with the magnetoresistive (MR) sensor element having said capability and suitability.
In accord with the objects of this invention there is provided a spin valve magnetoresistive (SVMR) sensor and a method for its fabrication. Said spin valve magnetoresistive (SVMR) sensor is of the bottom spin valve structure, which provides advantages in the reading of ultra-high density magnetic data over the more common top spin valve structure. Further in accord with the objects of this invention, said bottom spin valve structure of the present invention is fabricated so as to employ and embody the advantages resulting from the specular reflection of conduction electrons from certain material layers of the spin valve structure. Such specular reflection, when acting in concert with the spin dependent scattering of the GMR effect, produces further enhancements of the magnetoresistance ratio, Dr/r. Still further in accord with the objects of this invention, said specular reflecting, bottom spin valve structure comprises a CoFe-NiFe free layer and a TaO/NiCr specular reflection layer, which combination of layers and materials is experimentally found to produce a high output voltage, provide free-layer anisotropic properties that are superior to other ferromagnetic materials such as CoFe, provide a thermally more stable sensor element and thereby be the most suitable materials for satisfying the objects of the present invention. Finally, in accord with the objects of said invention said specular reflecting, bottom spin valve sensor is longitudinally hard-biased to maintain the proper magnetization orientation of the free ferromagnetic layer, using a continuous spacer exchange hard-bias structure which thereby eliminates the xe2x80x9cdead zonexe2x80x9d of the top-valve SVMR structure, which elimination is essential for the decoding of the narrow tracks in ultra-high density applications.